// SPDX-License-Identifier: GPL-2.0
#include <seminix/bug.h>
#include <seminix/kernel.h>
#include <seminix/div64.h>
#include <seminix/reciprocal_div.h>

/*
 * For a description of the algorithm please have a look at
 * include/linux/reciprocal_div.h
 */

struct reciprocal_value reciprocal_value(u32 d)
{
    struct reciprocal_value R;
    u64 m;
    int l;

    l = fls(d - 1);
    m = ((1ULL << 32) * ((1ULL << l) - d));
    do_div(m, d);
    ++m;
    R.m = (u32)m;
    R.sh1 = min(l, 1);
    R.sh2 = max(l - 1, 0);

    return R;
}

struct reciprocal_value_adv reciprocal_value_adv(u32 d, u8 prec)
{
    struct reciprocal_value_adv R;
    u32 l, post_shift;
    u64 mhigh, mlow;

    /* ceil(log2(d)) */
    l = fls(d - 1);
    /* NOTE: mlow/mhigh could overflow u64 when l == 32. This case needs to
     * be handled before calling "reciprocal_value_adv", please see the
     * comment at include/linux/reciprocal_div.h.
     */
    WARN(l == 32,
         "ceil(log2(0x%08x)) == 32, %s doesn't support such divisor",
         d, __func__);
    post_shift = l;
    mlow = 1ULL << (32 + l);
    do_div(mlow, d);
    mhigh = (1ULL << (32 + l)) + (1ULL << (32 + l - prec));
    do_div(mhigh, d);

    for (; post_shift > 0; post_shift--) {
        u64 lo = mlow >> 1, hi = mhigh >> 1;

        if (lo >= hi)
            break;

        mlow = lo;
        mhigh = hi;
    }

    R.m = (u32)mhigh;
    R.sh = post_shift;
    R.exp = l;
    R.is_wide_m = mhigh > U32_MAX;

    return R;
}
